Problem: $ { \left[\begin{array}{rr}{-1} & {0} \\ {1} & {1}\end{array}\right]+\left[\begin{array}{rr}{0} & {-1} \\ {2} & {-1}\end{array}\right] = {?} } $
To find the sum of two matrices, add the corresponding items between each matrix. ${\left[\begin{array}{rr}{-1}+{0} & {0}+{-1} \\ {1}+{2} & {1}+{-1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{-1} & {-1} \\ {3} & {0}\end{array}\right]}$